1. Field of the Invention
The present invention relates to fluorescent lamp ballasts and more specifically to a modular high frequency ballast used with various mains supply sources for driving different types of lamps, and for various operations of multiple lamps that are wired in series and in parallel.
2. Description of the Background of the Invention
Fluorescent lamp ballasts must serve a broad range of product requirements. These product requirements may include multiple lamp drives, for lamps ranging from one to four series and parallel wirings, instant and preheat starts, independent and non-independent operations, dimming and fixed light outputs, and operation on various mains supplies, including 120V, 277V and 240V sources. Furthermore, differing safety and Electro-Magnetic-Interference (EMI) standards in addition to driving of many different types of lamps are required for different products. As a consequence, a typical ballast producer must develop, manufacture, and stock a large number of individual product types ranging in excess of 200 Stock Keeping Units (SKUs), in order to operate as a full service supplier.
Shown in FIG. 1 is a typical multi-lamp dimming ballast system having the following components:
1. A power-factor-correction (PFC) block 2 for active power factor correction in response to a mains input 1.
2. A direct current (DC) BUS block 3 containing a large electrolytic capacitor for smoothing the voltage caused by the high-frequency load ripple current, as well as for providing a stable current supply during peak power events, e.g., ignition.
3. A regulated DC high-voltage 4 applied to the lamp inverter stage 5, which comprises a half-bridge Metal-Oxide-Silicon Field-Effect Transistor (MOSFET) power stage for driving a resonant tank circuit 6.
4. The resonant tank 6 may include multiple inductors and capacitors to achieve ballasting of more than one lamp 7. In addition a transformer may be included for isolation purposes.
5. Electrode heating is usually derived from an extra winding on an inductor a capacitor, or a separate transformer. In addition, a control integrated circuit (IC) 8 is used to generate the appropriate drive signal (level) 10 for the inverter (e.g.half-bridge configuration) 5 to achieve a desired output power level. The drive signal are gate signals supplied to the gates of the MOSFETs for turning the latter on and off. For better accuracy, the lamp output signals 9 may be sensed and compared with the control input to set the proper drive level 10.
Two problems are prevalent with the use of the lamp dimming ballast shown in FIG. 1 and described above. First, the existing systems are mostly dedicated to driving one configuration and one type of lamp. For example, in a dimming ballast, such as an Advance Mark VII type made by Advance Transformer Co. of Rosemont, Ill., from one to three lamps, series connected, may be driven. However, the values of the major tank components must be changed for each product model depending on the number of lamps and a specific lamp type. The series connection is limited by ignition voltage requirements for more than three lamps, additionally large reactive components are needed.
The same limitations hold for adjusting tank component values to the specific lamp type in the case of parallel lamp drive, such as the Philips Electronics High Frequency (HF)-R type. In addition, a balancing transformer is used in such configurations to compensate for component tolerances in low dim levels, which adds to the size and cost of the ballast.
An additional problem with existing dimming designs is the lack of Independent Lamp Operation (ILO). In the series connected situation, when a lamp is removed the entire chain is broken and all lamps are extinguished. Lamp removal, in a parallel connected lamp configuration, can activate a stop circuit that shuts down the inverter for safety reasons; again extinguishing all lamps.
To reduce the high costs of development and logistics of stocking a large number of individual product types, various approaches have been proposed, including introducing a more flexible ballast design concept, in order to significantly reduce the diversity of ballast designs. Many of these efforts have focused on the topology of a power network and specifically the combination of passive power elements.
What is needed is a modular high frequency fluorescent the lamp ballast for series and parallel lamp connections for one or more lamps. The ballast must be used for instant and preheat starts, independent and non-independent operations, dimming and fixed light outputs, and operation on various mains supply sources. Furthermore, the ballast must adhere to various safety and EMI standards required for different products and be able to drive many different types of lamps.